This invention relates to a magnetic disk apparatus, and more particularly to a magnetic disk apparatus such as, for example, a hard disk apparatus or a removable hard disk apparatus.
In a magnetic disk apparatus of a related art represented by a hard disk apparatus or a removable hard disk apparatus, efforts are directed to reduction of the access time and increase of the recording density.
The access time is a time required for a magnetic head to seek an aimed track to start recording or reproduction of a desired data area. In a magnetic disk apparatus of the type described above, the access time is represented by a sum of the seek time of the magnetic head and a latency time or rotational delay (typically, equal to one half of the time required for one rotation of the magnetic disk).
In particular, in a hard disk apparatus, a magnetic disk is driven to rotate in the condition of a constant angular velocity (CAV), so that, even if a seeking operation of the magnetic head is performed, the speed of rotation of the magnetic disk can be kept at a fixed rotational speed, thereby preventing an increase of the access time effectively. In the rotational driving in the condition of a constant angular velocity., the hard disk apparatus drives the magnetic disk to rotate at a high speed to assure a further reduction in access time.
Meanwhile, in order to increase the recording density, the linear recording density in a longitudinal direction of a track is increased and the track density is increased to increase the planar recording density. Incidentally, in recent years, products which have a recording density higher than 3 Gbit/inch2 have been placed on the market.
Where the magnetic disk is rotated in such a high speed of rotation and has such a high recording density as described above, the hard disk apparatus has a high data transfer rate. In particular., in a 3.5-inch-hard disk apparatus whose linear recording density is higher than 200 kbit/inch, the speed of rotation of the disk is approximately 5,400 rpm, and the maximum data transfer rate is higher than 170 Mbit/sec.
In a magnetic disk apparatus of the type described above, in order to allow recording and reproduction of moving pictures on the real-time basis, it is required to lower the speed of rotation of the magnetic disk to reduce the data transfer rate and reduce the power consumption as much. On the other hand, in order to allow downloading of moving pictures to a personal computer, it is required to raise the speed of rotation of the magnetic disk to raise the data transfer rate.
In this instance, it can be estimated that the requirements given above can be satisfied by switching the speed of rotation of the magnetic disk when necessary.
However, the magnetic disk apparatus in the related art described above is disadvantageous in that, if the speed of rotation of the magnetic disk is switched in this manner, then a sufficient tracking control characteristic and a sufficient capacity cannot be assured.
In particular, in recent years, data of moving pictures are compressed in accordance with the MPEG (Moving Picture Experts Group) 2 standards. According to the MPEG2 standards, moving pictures of a sufficiently high picture quality can be displayed at an average data transfer rate of 4 to 8 Mbit/sec.
To the previous hard disk apparatus of the related art, the data transfer rate is unnecessarily high for real-time recording and reproduction of video data of the type described above.
The data transfer rate depends upon the speed of rotation of the magnetic disk as described above. Thus, if the unnecessarily high data transfer rate can be reduced accompanied with the speed of rotation of the spindle motor lowered as much, then the power consumption and noise can be reduced accordingly.
However, if the speed of rotation of the magnetic disk is reduced in this manner, then the positioning accuracy of the magnetic head is deteriorated, resulting in reduction of the track density.
In particular, the hard disk apparatus in the related art forms servo areas at fixed distances on an information recording face of a hard disk and forms data areas, into which user data are to be recorded, between the servo areas. Several tens servo areas are provided for one track, and position information necessary for positioning the magnetic head such as of the head position and of the track number, synchronization patterns necessary for acquisition of the position information and other necessary information are recorded in the servo areas.
In the hard disk apparatus, in each servo area, a clock signal is synchronized first and then the position information of the magnetic head is acquired, and processing of tracking control and so forth is executed based on the position information. In particular, a positioning system for the magnetic head forms a servo loop to position the magnetic head. In the servo system, a closed loop control band Bsv is determined based on various conditions.
In particular, the hard disk apparatus must position the magnetic head at the center of a track so as not to be influenced by various external disturbances. Some of such disturbances appear in the inside of the hard disk apparatus are periodical and non-periodical components of whirling of the spindle motor, displacement of the position of the head by vibrations of the disk or vibrations of the head supporting arm, and so forth. The other disturbances come from the outside of the hard disk apparatus are various vibrations, an impact and so forth. Such disturbances are distributed in spectrum in a particularly low frequency region lower than the frequency of 1 kHz. Consequently, the closed loop control band Bsv of the head positioning servo system is required to be at least several hundreds Hz or higher in order to sufficiently suppress such disturbances as described above.
Generally, the control band Bsv of the positioning control system for the magnetic disk or the like is required to increase in proportion to the track density TPI to half power as given by the following expression (K. K. Chew, “Control system challenges to high track density magnetic storage”, IEEE Trans. Magn., Vol. 32, No. 3, pp.1799-1804, May 1996).Bsv∝(TPI)1/2  (1)
Consequently, the control band Bsv of the positioning control system for a magnetic disk or the like is required to be a band similar to that of an existing hard disk apparatus whose data transfer rate is unnecessarily high.
On the other hand, the servo system of the type described above is a closed loop sampling control system, and therefore, in order to assure the stability of the control loop, it is required that the servo sampling frequency fsv should be higher by more than 10 times than the control band Bsv. In particular, a relationship given by the following expression is determined, and at present, the servo sampling frequency fsv is set at least to several kHz.fsv>10Bsv  (2)
It is to be noted here that the servo sampling frequency fsv is the number of servo areas scanned by the magnetic head per unit time and is represented as the product of the servo area number Nsv in one circumferential track of the magnetic disk and the disk rotational speed R.fsv=Nsv·R  (3)
From the expressions (1) to (3) above, the following expression is obtained:Nsv·R=10Bsv∝(TPI)1/2  (4)
From the expression (4), it can be recognized that, if the rotational speed R of the magnetic speed is lowered to a necessary and sufficient level to record and reproduce information of moving pictures, then also the control band Bsv decreases.
If the control band Bsv decreases in this manner, particularly the performance of the magnetic disk apparatus in suppression of various disturbances is deteriorated, resulting in deterioration of the positioning accuracy of the head such that tracks cannot be formed in a high density.
One of possible solutions to this problem is to increase the servo area number Nsv per one circumference of the magnetic disk. However, the length of servo areas of a magnetic disk of the related art is comparatively long because it is required to perform processing for re-synchronizing the clock signal in each servo area and so forth. Therefore, if the servo area number Nsv is increased, then the recording capacity of the magnetic disk is deteriorated significantly as much.